The present application relates to an active material including silicon (Si) as a constituent element, to an electrode and a secondary battery that use the active material, and to a battery pack, an electric vehicle, an electric power storage system, an electric power tool, and an electronic apparatus that use the secondary battery.
In recent years, various electronic apparatuses such as a mobile phone and a personal digital assistant (PDA) have been widely used, and it has been demanded to further reduce the size and the weight of the electronic apparatuses and to achieve their long lives. Accordingly, as an electric power source for the electronic apparatuses, a battery, in particular, a small and light-weight secondary battery capable of providing high energy density has been developed.
In these days, it has been considered to apply such a secondary battery to various applications other than the electronic apparatuses. Examples of such applications other than the electronic apparatuses may include a battery pack attachably and detachably mounted on the electronic apparatuses or the like, an electric vehicle such as an electric automobile, an electric power storage system such as a home electric power server, and an electric power tool such as an electric drill. Further, applications other than the foregoing examples may be adopted.
Secondary batteries utilizing various charge and discharge principles to obtain a battery capacity have been proposed. In particular, a secondary battery utilizing insertion and extraction of an electrode reactant has attracted attention, since such a secondary battery provides higher energy density than a lead battery, a nickel-cadmium battery, and the like.
The secondary battery includes a cathode, an anode, and an electrolytic solution. The anode includes an active material (an anode active material) capable of inserting and extracting an electrode reactant. As the anode active material, carbon materials such as graphite have been widely used. Recently, since it has been demanded to further improve the battery capacity, using silicon has been considered. One reason for this is that, the theoretical capacity of silicon (4199 mAh/g) is significantly larger than the theoretical capacity of graphite (372 mAh/g), and therefore, the battery capacity is greatly improved thereby.
However, since silicon is intensely expanded and shrunk at the time of inserting and extracting an electrode reactant (at the time of charge and discharge), the anode active material is easily cracked mainly in the vicinity of a surface layer. In the case where the anode active material is cracked, a highly-reactive newly-formed surface (an active surface) is created, and therefore, the surface area (the reactive area) of the anode active material is increased. Thereby, a decomposition reaction of an electrolytic solution occurs on the newly-formed surface, the electrolytic solution is consumed for forming a coat derived from the electrolytic solution on the newly-formed surface, and therefore, the battery characteristics are easily lowered.
Therefore, in order to improve the battery characteristics, configurations of secondary batteries have been considered in various ways. Specifically, in order to improve cycle characteristics and the like, surfaces of particles such as silicon and silicon oxide are coated with graphite (for example, see Japanese Unexamined Patent Application Publication Nos. 2009-212074, 2011-090869, and 2011-076788). In order to improve initial efficiency and the like, a silicon-silicon oxide-lithium-based complex that is doped with lithium is used (for example, see Japanese Patent No. 4985949). In order to improve overcharge characteristics, overdischarge characteristics, and the like, a silicon oxide or a silicon salt each containing lithium is used (for example, see Japanese Patent No. 2997741).